It is well known in the art of dot matrix printing that the magnitude of the print voltage applied to the solenoids of the print head will vary and the variance depends on both static and dynamic factors. The static magnitude of the print voltage will vary from one DC power supply to another as a result of variations in component values and the degree of regulation against power line voltage fluctuations that is achieved. The main dynamic factor that affects the magnitude of print voltage is the instantaneous load factor, related to the number of solenoids being driven and thus the energy output requirement of the power supply.
Since variations in print voltage supplied to the solenoids will produce corresponding variations in dot print density and thus adversely affect print quality, it has been found necessary to provide automatic and dynamic compensation of the duration of the print drive pulse as a function of the print voltage magnitude. A number of prior art schemes for providing such dynamic compensation have been proposed.
FIG. 1 illustrates one prior art circuit arrangement to achieve such dynamic compensation. a TRIGGER signal of short duration (e.g. eight microseconds) on input terminal 10 is inverted in U1 and coupled through resistor R1 to the base of transistor Q1 to turn Q1 ON to discharge capacitor C1 and to drive the output of comparator U2 HIGH and thus initiate the SOLEN signal. Then when Q1 turns OFF, capacitor C1 begins to charge toward a positive voltage level at a rate dependent on the magnitude of V-PRINT and the overall series resistance value of resistors R2, R3, and P1, and the capacitance value of C1.
When the voltage level across capacitor C1 and thus the voltage on the minus terminal of comparator U2 reaches the level V-REF applied to the plus terminal, the output of comparator U2 goes low and the pulse drive signal SOLEN is terminated. As the magnitude of V-PRINT changes, the length of the SOLEN drive signal is automatically adjusted. A lower V-PRINT magnitude causes C1 to charge more slowly and thus lengthens the duration of the SOLEN signal at the output of U2. Accordingly the duration of SOLEN will vary inversely with the magnitude of V-PRINT. This is the functional relationship that is required to maintain uniform dot print density.
Because of manufacturing tolerances in circuit values, mainly the capacitance value of capacitor C1, it is necessary to adjust the resistance value of potentiometer P1 at the factory to achieve a sufficiently precise control over the relationship between the magnitude of V-PRINT and the duration of the SOLEN output signal.
Other approaches to such automatic compensation of the solenoid drive pulse duration are shown in Vollhardt U.S. Pat. No. 3,789,272 and Suzuki U.S. Pat. No. 4,514,737. While each of these prior art approaches achieves dynamic compensation of the drive pulse duration as an inverse function of V-PRINT voltage magnitude, it is believed that all of them require factory calibration of the circuit, e.g. the resistance value of an RC circuit that performs the timing function.